1. Field of the Invention
The present invention relates to a particulate sensor for detecting particulates contained in a gas under measurement.
2. Description of the Related Art
Patent Document 1 discloses an example of a particulate sensor which can detect particulates contained in exhaust gas. Specifically, Patent Document 1 discloses a particulate sensor which generates ions by means of corona discharge, and electrifies (or charges) particulates contained in exhaust gas by action of the ions, to thereby detect the amount of particulates contained in the exhaust gas.
The particulate sensor disclosed in Patent Document 1 has a sensor unit in which first through fifth insulative ceramic layers each formed of an insulating ceramic are laminated. In this sensor unit, first and second ground patterns are disposed between the plurality of insulative ceramic layers, and a discharge pattern is formed on the surface of a second ceramic layer by pattern printing. Electric power (2 to 3 kV, 100 kHz) for corona discharge is supplied to a distal end portion (discharge electrode) of a discharge pattern, whereby a corona discharge is produced between the discharge pattern and the second ground pattern (see FIG. 6 of Patent Document 1).
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2013-170914
3. Problems to be Solved by the Invention
In conventional particulate sensors, such as the particulate sensor of Patent Document 1, AC high voltage or pulsed DC high voltage has been applied between the electrodes in order to generate ions by means of corona discharge. The pulsed DC high voltage is obtained by half-wave rectifying AC high voltage or switching DC high voltage through on/off control, and contains a large AC voltage component.
However, in the case of gaseous discharge, such as corona discharge, generated by applying a high voltage, the use of AC high voltage or pulsed DC high voltage causes various problems. For example, the controllable frequency is restricted, expensive switching elements are needed, and control circuits become complex. This results in an increase in cost.
Meanwhile, in a sensor in which a ceramic laminate such as the sensor unit of Patent Document 1 is used, its insulating ceramic layer is a dielectric. Therefore, in the case where a discharge electrode is formed on the surface of the ceramic laminate, and a constant DC voltage whose voltage fluctuation is small; i.e., whose AC voltage component is small, is applied between the discharge electrode and a ground layer within the ceramic laminate, ions cannot be continuously generated by means of corona discharge.